Objective picture quality measures are important tools in image processing research and in off-line or real-time monitoring of image communications networks and broadcast installations. They are often used to assess the quality of image processing applications by comparing the input and output images. However, known methods are not readily applicable to the evaluation of standards conversion processes. By standards conversion process is meant a process converting an input signal in one format to an output signal in a different format, with each format usually being defined by a nationally or internationally recognised standard and specifying at least the number of the lines per field or frame and the number of fields or frames per second.
Standards converted output images generally have no temporally equivalent input images; and, the output images may have a different sampling structure from the input images. For example interlaced, standard-definition television with a field-rate of 50 Hz could be converted to progressively-scanned high-definition television with a frame-rate of 24 frames per second.
The picture impairments due to standards conversion often vary with time, typically depending on the relative temporal phasing between input and output images. In linear temporal conversion using weighted sums of differently-timed input pixels to create interpolated output pixels, slowly-moving objects become less sharply defined, and fast moving objects give rise to multiple images. In motion-compensated converters incorrect motion vectors and incorrect handling of occlusions can lead to the breakup of objects and the introduction of spurious edges.
The lack of direct temporal equivalence between input and output images and the particular nature of conversion impairments make the use of known objective quality evaluation techniques impractical. There is thus a need for an objective impairment measurement technique specifically appropriate to standards conversion and other processes involving the interpolation of an output image from at least two input images.